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In the context of laser physics, clustering is the tendency of laser-active ions in a solid-state gain medium to form clusters rather than to be randomly spread.
This is normally an undesirable effect, as it allows for energy transfer between laser ions which can seriously degrade the gain and power efficiency by processes which are called quenching.
For example, this can occur in erbium-doped gain media with high erbium concentration (e.g. in some erbium-doped fiber amplifiers).
Here, there are e.g. energy transfers involving two erbium ions which are initially in the upper laser level, where then one ion reaches the ground state and transfers its energy to the second ion.
The latter ion will quickly relax to the upper laser level, and effectively one of the two excitations is lost.
The (often weak) population of a high-lying state leads to some upconversion fluorescence.

The simplest way to avoid clustering is to keep the doping concentration low.
However, high doping concentrations are desirable in some situations, e.g. when a short fiber laser or amplifier must be constructed.
In that case, it is important to select a host material with a high solubility of the dopant.
For example, phosphate glasses allow for higher erbium concentrations without clustering, compared with silicate glasses.